1. Technical Field
The present invention relates to a method of forming a zinc oxide film, and more particularly, to a method of forming a p-type zinc oxide film containing dopants.
2. Description of the Related Art
Zinc oxide is a Group II-VI semiconductor having a hexagonal Wurzite crystalline structure, which is the same as those of GaN, ZnSe, ZnS, etc. The crystalline structure of zinc oxide has a degree of lattice mismatch of about 1.98% relative to gallium nitride that is a major material for light emitting diodes. Thus, ZnO has a high possibility of application to a heterogeneous stacking structure.
Also, zinc oxide has an optical band gap of about 3.37 eV, which is similar to the band gap of GaN amounting to 3.4 eV, so that it is available as a near-ultraviolet light source. Further, since zinc oxide also has relatively high defect-formation energy, it may have good properties if manufactured into optical devices. Furthermore, zinc oxide has a larger room-temperature exciton binding energy of 60 meV, which is about three times higher than those of GaN and ZnSe (24 and 19 meV). Thus, it can be expected that zinc oxide will provide highly efficient optical gain in exciton-based optoelectronic applications.
However, in order to apply zinc oxide to light emitting diodes and the like, deposition of a crystal thin film, implementation of a multiple quantum-well structure, and formation of a p-type semiconductor layer are important factors to be solved. Particularly in forming an existing p-type ZnO thin film, a source of zinc and a source of dopants are supplied at the same time, followed by annealing, thereby forming a p-type thin film. Particularly, the dopants include group V elements. The forming process may be metal organic chemical vapor deposition (MOCVD).
However, MOCVD causes poor crystallinity of zinc oxide and numerous defects in a crystal lattice. Moreover, dopant atoms are not uniformly distributed in the thin film, causing agglomeration or diffusion of the dopant atoms towards the surface of the thin film.
This is also due to a high escape rate of oxygen atoms in the crystal of ZnO. High diffusibility of oxygen intensifies crystal defects so that dopant atoms to be combined with oxygen are not uniformly distributed in the crystal structure.
Particularly, when the p-type ZnO thin film is formed using antimony as a dopant, a conventional MOCVD process is used.
FIG. 1 shows an image of a p-type zinc oxide thin film formed by a conventional MOCVD process.
Referring to FIG. 1, diethyl zinc (DEZn) and trimethyl antimony (TMSb) were used as precursors of zinc and antimony, respectively. In addition, a growing temperature is set to 600° C. while oxygen gas is supplied.
Antimony atoms are not crystallized due to low melting point, but can be easily evaporated. In addition, the interior of the film is agglomerated with ZnO and the outer surface of the film is formed of Sb-doped ZnO. This is caused by a phenomenon that antimony atoms are concentrated on the outer surface of the film due to diffusion of antimony. However, the p-type ZnO thin film having this structure has problems of insufficient carrier concentration and higher resistance. Thus, it is impossible to dope antimony in practice.